Could Subway Trains Generate Power with Regenerative Braking?

Photo: Flickr, CCCould Save Millions of Dollars Per YearVehicles without regenerative braking are throwing away a lot of energy every time they brake (movement is converted into heat via friction on the brake pads). Hybrid and electric car address that problem by capturing as much of that energy as possible and converting it into electricity, and the same might be about to happen to subway trains with the added twist that it would be tied to the (smart) power grid. The Southeastern Pennsylvania Transportation Authority (SEPTA) is experimenting with regen braking, a move that it estimates could slash its electricity bills by "up to 40 percent and generate millions of dollars a year." Read on for more details.Photo: Flickr, CC

The beauty of regen braking with trains is that you don't necessarily have to store the power onboard, which means that you don't have to carry the weight of a battery pack around:

A massive battery installed at one of the authority's substations will store electricity generated by the braking systems on trains (as the trains slow down the wheels drive generators). The battery will help trains accelerate, cutting power consumption, and will also provide extra power that can be sold back to the regional power grid. The pilot project, which involves one of 38 substations in the transit system, is expected to bring in $500,000 a year. This figure would multiply if the batteries are installed at other substations. (source)

The grid-tie is important; at certain times of day (during peak demand), it might pay more to allocate that extra electricity to the grid, especially if time-of-use rates are used. This could help cash-strapped transit agencies to make some extra money and help the environment by redeploying in a productive manner energy that would be lost as waste heat otherwise.

I even wonder if these battery stations couldn't be made with relatively inexpensive used EV batteries (they can still hold up to 80% of their charge even after they've been in an electric car).